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Meiosis and Sexual Life Cycles

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MEIOSIS AND SEXUAL LIFE CYCLES Chapter 13 REPRODUCTION Why do offspring closely resemble their parents and not unrelated individuals? heredity - passing of genes from ... – PowerPoint PPT presentation

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Title: Meiosis and Sexual Life Cycles


1
Meiosis and Sexual Life Cycles
  • Chapter 13

2
Reproduction
  • Why do offspring closely resemble their parents
    and not unrelated individuals?
  • heredity - passing of genes from parents to
    offspring
  • variation - inherited differences within a
    species
  • DNA - Nucleic acid polymer with bases A,C,T, and
    G
  • genes - units of heredity made of DNA found on
    the chromosomes

3
Asexual Reproduction
  • What occurs in asexual reproduction?
  • Single individual is the parent which copies all
    of its genes into an identically cloned offspring
  • Limited variation with asexual reproduction
  • What nuclear division is involved in asexual
    reproduction?

4
Sexual Reproduction
  • In order for 2 parents to combine their genes
    into one offspring, what must their genetic
    contribution be?
  • Each parent must contribute 1/2 of their genes to
    the offspring
  • How does the process ensure that the offspring
    receives the correct genes from each parent?

5
Vocabulary of genetics and reproduction
  • somatic cell - any cell other than sperm or egg
  • Human somatic cells have 46 chromosomes
  • karyotype - display of individuals somatic cell
    metaphase chromosomes
  • homologous chromosomes - chromosomes that have
    same centromere position, same genetic loci, and
    same length
  • How, then, are homologous chromosomes different
    from one another?

6
  • homologous chromosomes have different alleles.
  • allele - expression of a gene
  • autosome - non-sex chromosome
  • sex chromosome - dissimilar chromosomes that
    determine an individuals sex
  • How many autosomes and how many sex chromosomes
    are there in a normal human?
  • 44 autosomes, 22 pairs
  • 2 sex chromosomes, 1 pair
  • How many homologous pairs of chromosomes?

7
  • diploid - condition in which cell contains 2 sets
    of chromosomes abbreviated 2n
  • haploid - condition in which cells contain 1 set
    (no homologous pairs) of chromosomes abbreviated
    n
  • gamete - haploid reproductive cells, human
    gametes have 23 chromosomes
  • fertilization - union of 2 gametes
  • zygote - diploid cell that results from fusion of
    gametes
  • What sort of nuclear division must occur to
    produce haploid gametes?

8
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9
Meiosis
  • Must have a stage that results in the reduction
    of chromosome number
  • In meiosis, the diploid parent cell produces
    haploid gametes

10
Interphase I
  • What happens in interphase I?
  • Chromosomes replicate
  • centriole pairs in animal cells also replicate
  • In what way is interphase I from meiosis I
    different from interphase of mitosis?
  • There is no difference

11
Prophase I
  • Synapsis occurs
  • homologous chromosomes come together as pairs
  • chromosomes appear as tetrads with 4 attached
    sister chromatids
  • chiasmata form, they are sites where homologous
    non-sister chromatids cross over
  • 90 of meiosis is spent in prophase I

12
Metaphase I
  • tetrads align at metaphase plate
  • synaptic pairs are aligned so that centromeres of
    homologues point to opposite poles
  • each homologue is attached to kinetochore
    microtubules emerging from opposite poles, so
    that the 2 homologues are destined to separate
    during anaphase I

13
Anaphase I
  • homologous chromosomes are moved to opposite
    poles by spindle apparatus
  • sister chromatids remain attached at the
    centromeres
  • How does this differ from anaphase of mitosis?

14
Telophase I
  • spindle apparatus continues to separate
    homologous pairs of chromosomes
  • each pole has a haploid set of chromosomes
  • cytokinesis occurs and daughter cells are formed.
  • What is the chromosome number in each daughter
    cell?
  • haploid

15
  • What has been accomplished by the end of meiosis
    I?
  • homologous pairs of chromosomes have separated
  • Why isnt meiosis over, if haploid daughter cells
    have already been formed?
  • We still need to separate sister chromatids

16
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19
Genetic Variation
  • How do independent assortment and random
    fertilization contribute to genetic variation?
  • orientation of homologous pairs of chromosomes
    with regard to each-other is random (metaphase I)
  • there is a 50/50 chance that a particular
    daughter cell will receive a paternal or maternal
    chromosome

20
Genetic Variation
  • There are 2n possible combinations of maternal
    and paternal chromosomes
  • In humans there are 223 possible different
    combinations or 8 million
  • Random fertilization has the random combining of
    egg and sperm. Each egg and sperm has one in 8
    million different possibilities. Thus the zygote
    can have one in 64 trillion possible diploid
    combinations.
  • This number hasnt even accounted for crossing
    over, which has a random contribution to genetic
    variation.

21
Genetic Variation in all organisms
  • In asexually reproducing organisms, mutations are
    the primary source of genetic variation
  • In sexually reproducing organisms meiosis
    accounts for most of the variation
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